In this work, we demonstrate the use of 3D-printed molds for fabricating PDMS-based microfluidic devices. 3D Printing allows for the fabrication of molds that are not monolithic in structure, and therefore represents a significant improvement over the capabilities of standard soft lithography; with 3D-printed molds, we can fabricate most features commonly generated by soft lithography in addition to formerly difficult features such as domes and variable-sized channels. Furthermore, we demonstrate that this technique can be used to generate microfluidic devices molded on both sides - which allows for single-step generation of features like vias, thin membranes, and membrane valves - and can be easily adapted to generate multi-layer microfluidic structures.